Some turbochargers are provided with a hydraulic servo device. The hydraulic servo device includes a servo piston that is movably disposed to a device main body, and a stroke sensor for detecting the amount of movement of the servo piston with respect to the device main body, and functions to change the area of an exhaust passage of the turbocharger by the movement of the servo piston. The stroke sensor includes a moving element that is provided to the servo piston, and a stator that is provided to the device main body, and detects the amount of movement of the servo piston with respect to the device main body by detecting movement of the moving element by the stator. According to a turbocharger provided with this type of hydraulic servo device, the supply flow rate of exhaust gas to a turbine is changed by changing the area of the exhaust passage. Accordingly, for example, in a low revolution area of an engine, the supply flow rate of exhaust gas to the turbine may be increased by reducing the area of the passage, and the rotation energy of the turbine is increased, and the charging performance of a compressor may be increased (for example, see Patent Literature 1).